db.student2.drop();
db.student2.insert([
    {name:"tom", age:18, class:"a"},
    {name:"bill", age:16, class:"b"},
    {name:"jenny", age:17, class:"a"},
    {name:"rose", age:20, class:"c"},
    {name:"jerry", age:19, class:"a"},
    {name:"elena", age:18, class:"a"},
    {name:"cinderlla", age:16, class:"a"},
    {name:"belle", age:19, class:"a"},
    {name:"rapunzel", age:18, class:"a"},
    {name:"snow white", age:16, class:"a"},
    {name:"ariel", age:16, class:"a"},
    {name:"mulan", age:15, class:"a"},
    {name:"aurora", age:18, class:"a"},
    {name:"jasmine", age:19, class:"a"},
    {name:"tiana", age:17, class:"a"},
    {name:"merida", age:17, class:"a"},
    {name:"pocahontas", age:18, class:"a"},
]);